I've just copy-pasted this and am pretty sure this is the correct answer: There are five parameters necessary to create any oil and gas accumulations:1. Source- The geologic history and biologic history of an area must have produced sufficient organic material necessary to produce oil and gas.2. Reservoir- The better oil and gas reservoirs have excellent porosities and permeabilities that permitted the hydrocarbons to be concentrated from the very beginning, and then to be maintained once the hydrocarbons entered the interstitial spaces.3. Seal- The reservoir in which the hydrocarbons are entrapped must be sealed with impervious rocks that keep the bulk of hydrocarbons from escaping.4. Structure- The position of the Reservoir and the Seal must be such that the Seal lies above, or laterally against, the Reservoir and the Reservoir is oriented to concentrate the hydrocarbons in the upper portion of the Reservoir. The integrity of the Reservoir must then keep the bulk of hydrocarbons from escaping.5. Timing- All of these parameters must have occurred in the proper sequence to have generated hydrocarbons from the organic source materials and to have effected a "trap" of hydrocarbons